1. Field of the Invention
The present invention relates to a method of and an apparatus for generating color conversion data to convert color image data C, M, Y, K which are used to produce a printed material into color image data Cxe2x80x2, Mxe2x80x2, Yxe2x80x2, Kxe2x80x2 which are used in a color output apparatus to equalize the colors of the printed material with the colors of a color image produced by the color output apparatus.
2. Description of the Related Art
Before producing a colored print with a color printing machine such as a rotary press, a color proof image is generated by a color output apparatus such as a DDCP (Direct Digital Color Proofing System), and examined and corrected. Such a color output apparatus makes it unnecessary to use platemaking films and press plates for proofreading, and hence makes the proofreading process highly efficient.
Color output apparatus for producing color images based on color image data of four colors C, M, Y, K need to effect a color conversion process on given color image data of C, M, Y, K in order to produce color images which represent the colors of final prints highly accurately.
Specifically, predicting the colors of final prints with a color printer needs various conversion tables, including a printing condition correcting conversion table for converting color image data in view of printing conditions (e.g., the type of the print paper used, and the type of inks used) of a color printing machine, a standard color conversion table for making standard color corrections depending on the output principles (e.g., halftone dot modulation or density modulation) of the color printer and the color printing machine, independently of the printing conditions, and a calibration conversion table for correcting individual characteristics of the color printer, the environment in which the color printer is used, and characteristic changes due to aging of the color printer. Using these conversion tables, it is possible to produce color images easily which are highly accurately predictive of the colors of final printed materials.
When dealing with color image data of four colors C, M, Y, K, each of the printing condition correcting conversion table and the calibration conversion table can easily be determined as a one-dimensional conversion relationship between monochromatic colors. However, because the standard color conversion table represents a highly precise four-dimensional conversion relationship between four colors, various problems arise in the determination of the standard color conversion table, as described below.
Heretofore, a standard color conversion table is produced as follows: A reference print paper and reference inks are selected, and a test chart composed of color patches is produced by a color printing machine of desired output principle using the reference print paper and the reference inks, after which the colorimetric values of the test chart are measured. Another test chart is produced by a reference color printer from the color image data of C, M, Y, K based on which the test chart has been produced by the color printing machine, and the calorimetric values of the test chart are measured. A table capable of equalizing the colorimetric values of the test charts is determined as the standard color conversion table.
The standard color conversion table, the printing condition correcting conversion table, and the calibration conversion table may be combined into a desired conversion table for incorporation into a color printer. However, since the amount of data of these conversion tables is huge, it takes a considerable period of time to combine these conversion tables.
If there are 11 items (10 halftone dot % intervals) of each of the color image data of C, M, Y, K, then the total number of color patches of a test chart required to generate a standard color conversion table will be 114=14641. It also takes a considerable period of time to measure the colorimetric values of all these color patches.
It is a major object of the present invention to provide a method of and an apparatus for generating a minimum required amount of color conversion data, which requires a relatively small storage capacity for storage, within a relatively short period of time, the color conversion data being capable of highly accurate color conversion at high speed.